This is a very good skin, matching well with the iTunes for windows. I have a few trivial suggestions. On the contact list, I really like the alternating colours (just like mac and itunes for win), but maybe the rows could be a little thinner?

I realy like the buttons at the bottom, but the buttons on the conversation window don't match those in the main window.
Good work and I look forward to a final release!

Looks good so far. I'd recomment replacing those typical mac buttons on the top left with the regular iTunes 'X' for close, and the other icons. Would look a bit better and also there's tons of those mac buttons floating around now anyway, vary it up a bit.